Electrochemical corrosion is a process in which a metal atom oxidizes and loses electrons. In corrosion parlance, the location at which metal atoms lose electrons is called the anode and the location where the electrons are transferred is called the cathode. Localized corrosion within a base metal involves the creation of actively corroding anode areas separated from the non-corroding cathode areas. Thus, as the base metal oxidizes at the anode, forming a pit in the anodic area, the electrons left behind flow to the cathode. This flow of electrons forms essentially an electrical circuit. As such, when the circuit is closed and an electrical potential difference exists between the anode and the cathode, an electrical current flows between the anode and the cathode.
Gasification is a partial oxidation process that transforms a hydrocarbon feedstock, such as coal, into synthetic gas (syngas), which may then be used as a cleaner, more environmentally friendly means of generating power. Typically, the gasification process is performed in a gasifier, often referred to as a gasification unit. Within the gasifier, a chemical reaction occurs when the hydrocarbon feedstock is mixed with oxygen and steam under high pressure and heat. For example, the feedstock, such as coal, may be chemically broken apart by the pressure and heat within the gasifier, resulting in chemical reactions that produce both hydrogen and carbon monoxide, the primary components of syngas. This syngas may then be cleaned and supplied to a turbine system, such as the combined cycle turbine system of an integrated gasification combined cycle (IGCC) power plant, to generate electricity.
During the gasification process, various corrosive agents are handled within the gasifier. For example, highly corrosive gases, such as ammonium chloride, hydrogen sulfide and hydrogen chloride, are often produced during the gasification process. Theses high temperature, corrosive gases diffuse throughout the gasifier and, particularly, contact the inner wall of the pressure vessel of the gasifier. Often, due to material constraints on the pressure vessel, the wall surface temperature of the vessel is below the gaseous salt dew point of many of the corrosive gases. As a result, the corrosive gases contacting the vessel wall condense and drip down the face of the wall. These condensing gaseous salts and other corrosives continuously coat the vessel wall, which may lead to corrosion of the pressure vessel.
Currently, one of the only means for monitoring most industrial equipment exposed to high temperature corrosion, such as chemical reactor vessels, is shut down and inspection of the equipment. In the case of gasifiers, downtime can be very costly. Additionally, inspections often necessitate dismantlement of at least a portion of the gasifier, which requires further downtime and expense. Other options may be available to a gasifier or IGCC operator, such as adjusting certain operating parameters of the gasifier, but it is often very difficult to predict when this action should to be taken. Thus, unnecessary expenditures are made when adjustments are made too frequently or too infrequently.
Accordingly, a system and method for online monitoring of corrosion of a metal component, such as a pressure vessel, would be welcome in the technology.